1. Field of the Invention
The present invention relates to an embedded multi-layer circuit board and a noise suppression method, and particularly to an embedded multi-layer circuit board and a noise suppression method employing a periodical structure.
2. Description of the Prior Art
With the rapid development of modern technologies, the operating speed of the digital circuit and clock frequency become higher and higher, and the composition of the analog circuit is getting more complicated, and the frequency bands with which the RF (Radio frequency) circuit operates are increased. Moreover, the current trend is to make the whole printed circuit board smaller, and even develops toward the compact system package. Thus, a plurality of transmission wires for transmitting different signals will be integrated into a tiny space. Since most of the signals exist between the ground boards in the multi-layer circuit board, the transmission mode between the ground boards will cause the coupling noise. For example, when a signal is transmitting on a transmission wire, a part of the energy will be coupled with the transmission mode between the ground boards, and a part of these modes will be coupled with another transmission wire, and the other part will travel to the edge of the ground board; then the energy will be radiated to the external space. Since a plurality of chips share the same power layer, the resonance of the power layer will be activated when the operating frequency of the chip exceeds the resonance frequency of the power layer, and thereby the power layers originally being of equal potential become the power layers with greatly fluctuated potentials, such that the chip may not be operable normally, or may even not operable.
There are many methods for suppressing the coupling noise, but most of them are used in the case that only one side has a ground plane. Although the same methods can be applied to the embedded circuit, the effect is not significant. As for the noise suppression in the power layer, the commonest method is using the decoupled capacitor. The decoupled capacitor may provide the path of short-circuit under natural frequency of vibration, so as to suppress the resonance mode of the power layer. However, due to the limitation of parasitic inductance, it is difficult to raise the natural frequency of vibration. Another method is using a periodical structure. The periodical structure can produce the electromagnetic bandgap to suppress the transmission mode of the power layer. The structures may be divided into two types according to their profiles. The first type is a mushroom-shaped structure, which has the defect that the bandwidth of the electromagnetic bandgap is not large, and it requires an additional space of two layers to be applied in the embedded power layer. The second type is a plane structure. Although such structure provides larger bandwidth of the electromagnetic bandgap, it cannot produce the electromagnetic bandgap if directly applied in the embedded power layer.
Therefore, there is a pressing need for an embedded multi-layer circuit board and a noise suppression method employing plane-type periodical structure, in which the mode between ground boards can be transmitted without an electromagnetic bandgap.